The present invention relates to a heat generator that generates heat by shearing viscous fluid.
A typical heat generator used as an auxiliary heat source for a vehicle has a housing and a rotor. The rotor, which has a specially designed shape, is rotated to shear silicone oil filling the housing, to generate heat. For example, Japanese Unexamined Patent Publication No. 2-246823 discloses a rotor having labyrinthine grooves. Japanese Unexamined Utility Model Publication No. 3-98107 discloses a rotor having multiple fins. The applicant company has also proposed a heat generator having a disk-shaped rotor.
A conventional disk-shaped rotor is made by machining carbon steel, such as S45C, and has a hole in the center. The diameter of the hole is slightly smaller than that of a drive shaft to which the rotor is to be fitted. The rotor is secured to one end of the drive shaft by press fitting the drive shaft into the hole of the rotor. When machining the rotor, a boss is formed about the hole. The boss is axially longer than the rest of the rotor. The boss increases the contact area between the rotor and the shaft thereby securely fixing the rotor on the shaft. The greater the force acting on the contact area, due to the press fit, the less the connection between the rotor and the drive shaft is affected by temperature changes in the heat generator.
However, machining the rotor from steel is difficult and burdensome, thus increasing costs. Heat generators having rotors as described above are therefore not suitable for mass production. Thus, a relatively thin steel plate made of SPCC or SPHC has been tested as a material for a rotor. That is, a plate made of SPCC or SPHC was deep-drawn into a rotor. However, steel plates that are used in presswork have a relatively weak tensile strength. The rotor is therefore hardened immediately after being pressed for improving the tensile strength of parts in the rotor (especially, the boss). The hardening improves the tensile strength of the rotor. The rotor is therefore securely fixed to the drive shaft.
However, hardening is very costly. Further, rotors are often deformed by hardening. Therefore hardened plate often needs to be processed to correct its deformation. Approximately half of the manufacturing cost of a heat generator can be spent on hardening of the rotor and the process thereafter. Thus, as far as cost saving is concerned, there is no reason to manufacture the rotor by pressing instead of by machining.